This invention relates generally to x-ray sources, and more particularly to the electronic control of rotating anode microfocus x-ray tubes to afford extended anode life.
X-ray radiography techniques such as computerized tomography (C.T.) and digital fluoroscopy (D.F.) may be used to advantage in many industrial applications, such as, for example, the non-destructive testing and inspection of parts. Such applications often require high intensity, high power x-ray sources capable of substantially continuous operation with very small focal spot sizes. For example, the inspection of parts formed of high atomic number materials, such as superalloy turbine blades for high performance aircraft engines, require x-ray sources capable of operating at voltages of the order of 400-500 kilovolts (kV) and high power levels of the order of tens to hundreds of kilowatts (kW) with focal spot sizes of the order of 1-10 mils. Since conventional fixed anode x-ray tubes have limited power dissipation capability, it is desirable to employ a rotating anode x-ray tube for such applications. Copending U.S. application Ser. No. 623,903, filed June 25, 1984, assigned to the assignee of the present invention, discloses a high intensity microfocus rotating anode x-ray tube that is particularly well adapted for such purposes.
A rotating anode x-ray tube typically comprises an evacuated enclosure housing an electron beam source, an electron beam deflection and focusing system, and an anode which is rotated at rather high speeds by means of a shaft which extends through a rotating seal in the wall of the enclosure. The rotating anode, which comprises an x-ray emissive material such as tungsten, is a relatively large and expensive element which is precisely machined and balanced. The electron beam is focused onto the rotating anode. As the electrons penetrate the anode, they give up their energy, causing heating of the anode, and emit x-rays. The large temperature gradients and temperature cycling to which the anode is subjected causes microfractures or cracks to develop in the anode early in its use cycle, and crack growth due to repeated thermal stresses ultimately makes the anode unusable, necessitating its replacement.
Thermal stresses are a significant contributing factor to fatigue and cracking of an x-ray tube anode. Because of the large temperature gradients and temperature cycling to which they are subjected, the anodes of high power x-ray tubes experience large thermal stresses which cause fatigue and short life. The thermal stresses are the greatest when the x-ray tube is first turned on. As the electrons penetrate the cold anode and give up their energy to produce x-rays, they produce highly localized and rapid heating of the anode under the electron beam spot and, accordingly, subject the anode to high temperature gradients. Tungsten, for example, which is a typical x-ray emissive anode material, is quite brittle and susceptible to cracking when its temperature is below a transition region temperature of the order of 1200.degree. C., at which it becomes ductile.
Because the rotating anode is an expensive element, and its replacement is time consuming, it is desirable to extend the anode life as much as possible. This has led to the development of alloys having microstructures and properties which resist fatigue and extend anode life. Although such developments are useful for extending anode life, it is desirable to provide other ways of extending anode life which are simpler and more easily implemented, and it is to this end that the present invention is directed.